The invention relates to a method determining an instantaneously optimum pressure of the brakes of a trailer or semitrailer connected to a tractor and, more particularly, to such a method for the purpose of adjusting the coupling force occurring between the tractor and the trailer or semitrailer to conditions instantaneously present.
DE 3,901,270 AS discloses a braking device in which during the first braking, the actual pressure value at the trailer is varied until the measured drawbar force vanishes. A braking pressure correction value is derived from the difference between the prescribed desired pressure value and the actual pressure value. No information or suggestion is provided, however, as to how the braking pressure correction value is to be derived.
Furthermore, DT 2,164,352 B2 discloses a method in accordance with which the medium braking pressure on the wheels of a motor vehicle trailer is regulated as a function of the coupling force between the tractor and the trailer. In this case, the medium braking pressure is regulated such that the coupling force vanishes or becomes as small as possible. It is taken into account whether the braking process is stable or unstable, i.e. whether, in the case of the occurrence of a positive coupling force, a greater retardation of the trailer is produced and thus the coupling force become smaller given an increase in the medium braking pressure or whether the wheels of the trailer lock, and thus the coupling force increases once again, give an increase in the medium braking pressure.
It could be regarded as disadvantageous in the previously known methods that braking pressure is not set to an adjusted value until the occurrence of a coupling force, as a result of which under certain circumstances driving comfort could possibly be impaired during the braking process.
It is an object of the present invention to improve upon known methods such that during a braking process as much driving comfort as possible is guaranteed simultaneously with driving safety that is as high as possible during the braking process.
In a general method of determining an instantaneous optimum braking pressure for a trailer or semi-trailer connected to a tractor, this object has been achieved according to the present invention by providing that, during at least one braking process in which the coupling force has been set, equal to its desired value, the assignment of pressure of the brakes of the tractor to the pressure is determined, taking account of the certain parameters. A conclusion is drawn from the determined assignments in the range of small values of the pressure on the assignments in the range of larger values of the pressure, and during subsequent braking processes, the target value of the pressure of the brakes of the trailer is determined at least indirectly from the assignment of the pressure of the brakes of the tractor to the pressure.
By contrast with known methods, advantages of the present invention include that, when the method according to the invention is applied, the desired value of the coupling force can be achieved very quickly.
In the method according to the present invention for determining an instantaneously optimum braking pressure for a trailer or semitrailer connected to a tractor, a first value is derived for an instantaneously optimum braking pressure by assigning to the instantaneous position of the braking value sensor (brake pedal) a value, derived from earlier stationary braking processes, of the braking pressure of the trailer or semitrailer as a target value of the braking pressure of the trailer or semitrailer. It is possible in this way to take account of differences arising in the individual brake systems, which can be based in different designs of the brake systems of the tractor and trailer or semitrailer or in different conditions with regard to ageing.
A stationary braking process is derived from the condition that over a relatively long period of time the coupling force is equal to zero or equal to the desired value of the coupling force. Deviations within a prescribed threshold value, which arise, for example, from measurement inaccuracies, likewise are recognized as a stationary braking process. In this case, this relatively long period of time can, in particular, be on the order of magnitude of approximately 0.5 s. A desired value of the coupling force is that is not equal to 0 can advantageously be used when the trailer is a central axis trailer. In the case of such a road train, a component of the braking force for the trailer, which corresponds to the supporting force on the tractor, must be accepted by the tractor. In the same way, the method can also be used for semitrailer trains. Only the term "trailer" will be generally used in the following description, although all other possibilities of composing a road train that are indicated here are understood to be encompassed by the present invention.
Both changes in the composition (that is to say a change in the trailer attached to the tractor) of the tractor-trailer combination and changes in the loading of the tractor-trailer combination are advantageously taken into account by resetting the stored assignments in the case of changes in the composition and/or changes in the loading. Changes in the composition of the tractor-trailer combination, as well as changes in the loading can be derived in this case, for example, from a relatively long stationary period or an engine standstill. The duration of this relatively long standstill can be fixed in this case at, say, 2 minutes.
After the braking pressure of the trailer assigned to the instantaneous position of the braking value sensor has been set, this braking pressure can then additionally be regulated so that the coupling force present between the tractor and the trailer reaches its desired value. The assignment of the position of the braking value sensor to the braking pressure of the trailer can then be used as the assignment for a target braking pressure for a specific position of the braking value sensor if this regulation were to lead in turn to the occurrence of a stationary braking process.
Various indices will be used below for the pressures in the braking system. The pressure designation P.sub.ALB designates the pressure upstream of the automatically load-dependent braking force valve (ALB valve) of the tractor, and the pressure designation P.sub.KKB designates the pressure at the coupling head brake between the tractor and the trailer, and thus designates the braking pressure of the trailer.